The embodiments described herein relate generally to electric/hybrid vehicles, and more specifically, to methods and systems for controlling operation of a vehicle.
Control devices used in vehicles have conventionally been proposed to provide an electric motor and a planetary gear type speed reducer for each of two rear wheels. The planetary gear type speed reducer is provided with a sun gear, a plurality of planetary gears that mesh with the sun gear, a planetary carrier that supports the planetary gears, and a ring gear that meshes with the outer periphery side of the planetary gears.
The output shaft of the electric motor is configured to output to the sun gear of the planetary gear type speed reducer. The ring gear is supported to freely rotate in a planetary gear type speed reducer case that covers the planetary gear type speed reducer. A one-way clutch and a hydraulic brake that controls the rotation of the ring gear are provided in a space between the outer radial side of the planetary gear type speed reducer and the planetary gear type speed reducer case. The one-way clutch is provided to lock the rotation of the ring gear when reverse rotational directional torque acts on the ring gear when the rotational direction of the sun gear is in a normal rotational direction as a vehicle advances forward. The planetary carrier is connected to the output shaft of the planetary gear type speed reducer, and the output shaft is connected to the drive shaft provided between the rear wheels. The drive shaft is configured to be connected to the rear wheels through the rear wheel axle.
In this type of configuration, the driving force of the electric motor is input into the sun gear of the planetary gear type speed reducer, and a reduced driving force is output from the planetary gear type speed reducer through the planetary carrier. When the vehicle is running forward by the driving force of the electric motor, the ring gear is locked by the one-way clutch, and thus, the driving force of the electric motor is output to the drive shaft. The output to the drive shaft is conveyed to the rear wheels through the rear wheel axle.
However, in this type of vehicle, a large amount of torsional torque is generated in a drive shaft provided between the rear wheels and the output shaft of the planetary gear type speed reducer when the rear wheels lock due to a driver suddenly pressing on the brake when the vehicle is running forward by the driving force of the electric motor.
FIG. 18A shows a vehicle and an expanded view of a vehicle running gear and wheel. More specifically, FIG. 18A illustrates a case where the vehicle is running forward by the driving force of an electric motor during normal driving. In the figure, an output shaft of a rotor 15 included in the electric motor is connected to an input shaft of a planetary gear type speed reducer 12. When a vehicle 3 is running forward by the driving force of the electric motor, the rotational speed of the rotor 15 of the electric motor is controlled at a rotational speed several times greater than that of a rear wheel Wr to accommodate the reduction ratio of the planetary gear type speed reducer 12. Control of the electric motor during normal driving is performed from the perspective of fuel consumption improvement of the vehicle overall and from the perspective of ride quality improvement for the driver based on the assumption that vehicle control is being performed according to operations by the driver.
The output shaft of the planetary gear type speed reducer 12 is connected to one end of a drive shaft 71. Further, another end of the drive shaft 71 is connected to the axle (not illustrated) provided for the rear wheel Wr.
In this type of configuration, when the vehicle 3 is running forward by the driving force of the electric motor, the driving force from the rotor 15 of the electric motor is transferred to the drive shaft 71 in a decelerated state via the planetary gear type speed reducer 12, and this is transferred to the rear wheel Wr. Therefore, the vehicle 3 advances forward by the driving force of the electric motor.
Meanwhile, FIG. 18B illustrates “locking of the rear wheels Wr due to sudden braking” when the driver at the time of normal driving as illustrated in FIG. 18A applies the brakes suddenly. Torsional torque is generated in the drive shaft at the time of sudden braking.
In FIG. 18B, the rear wheels Wr are locked. Moreover, as stated here, “locking of the rear wheels Wr due to sudden braking,” also includes cases in which the rear wheels Wr lock for other reasons or when a similar situation to this occurs. Therefore, it is not limited to when sudden braking is applied but also includes an antilock brake system (ABS) activating on rear wheels Wr when brakes are applied on a low coefficient of friction (μ) road, rear wheels Wr locking due to side brake operation, and rear wheels Wr locking due to a parking brake being operated.
With respect to this, because normal running control has been undertaken so far in the electric motor, torque is generated by the rotor 15 as the rotor 15 tries to continue rotating in the same direction as before due to intrinsic inertia (hereinafter, torque generated by the intrinsic inertia held by a rotating body such as the rotor 15 will be referred to as “inertia torque”).
FIG. 19 is a diagram for explaining the mechanism that generates excess torque on the drive shaft at the time of sudden braking. In the figure, when the vehicle 3 is running forward by the driving force of the electric motor, the driver applies sudden braking at a time Ta resulting in the rear wheels Wr of the vehicle 3 being locked.
The rotor 15 of the electric motor is rotating at high speed at the time Ta. Therefore, because the rotor 15 drives the rear wheels Wr that are attempting to lock due to the inertia torque of the wheels themselves, torsional torque is generated in the drive shaft 71. Because a planetary gear type speed reducer 12 is provided between the rotor 15 and the drive shaft 71, the torque is increased according to the reduction ratio of the planetary gear type speed reducer 12, and as a result, excess torque is generated on the drive shaft 71 that is connected to the output shaft of the planetary gear type speed reducer 12. Furthermore, because the sun gear, planetary gears, and planetary carrier, which are component parts of the planetary gear type speed reducer 12, are also rotating at high speed at the time Ta, inertia torque due to their own inertia is also added to the drive shaft 71 making the excess torque on the drive shaft 71 even larger.
When considering durability of the drive shaft 71, this excess torque is problematic.
Moreover, after such excess torque is generated, the intrinsic elasticity in the drive shaft 71 generates torque on the drive shaft 71 to rotate the rotor 15 in the opposite direction. Therefore, the rotor 15 momentarily rotates in the reverse rotational direction until the vehicle speed and the rotational speed of the motor converge together at zero at a time Tb thereafter.
Conventionally, the generation of excess torque in the type of drive shaft 71 described above has been handled by increasing the strength of the drive shaft 71. However, increases in the weight and dimension of the components cause fuel consumption of the vehicle 3 to worsen. Further, increasing the size of the components leads to restrictions in the layout of the drive train.